1. Field of the Invention
The present invention relates to an image processing technique, in particular, a tone-correction technique for digital image data.
2. Description of the Related Art
Currently, the household penetration of digital cameras in Japan is more than 50%, and the act of “taking a photo with a digital camera” is very common. In the case where a user takes a photo with a digital camera outdoors, the luminance range of a scene that is to be photographed may be wider than a photographable luminance range. At that time, tone information of a subject that is outside the photographable luminance range cannot be recorded, and, thus, a highlight or shadow detail loss occurs. For example, in the case where a person is photographed outdoors in fine weather, if the exposure is matched to the person, a highlight detail loss may occur in the sky or clouds in the background, or a shadow detail loss may occur in a tree shade. However, as typified by the Retinex model in the document “Edwin H. Land and John J. McCann, “Lightness and Retinex Theory”, Journal of the Optical Society of America, Vol. 61, Num 1, 1971”, human vision more sensitively detects the reflectance of an object, and can sense the tone both in light places and in dark places. Accordingly, the impression of a certain scene to the eye and the impression of the same scene in a photographed image may differ, which causes dissatisfaction to digital camera users.
One of the techniques for solving this sort of problem is a high dynamic-range (HDR) technique. The HDR technique is configured roughly from the HDR capture technique and the dynamic-range compression technique. The HDR capture technique is a technique for recording tone information of a luminance range in which a highlight or shadow detail loss has occurred, by increasing the photographable dynamic range. For example, there is a method in which images photographed with a plurality of exposures are combined. Hereinafter, an image captured by this HDR capture is referred to as an HDR image. Meanwhile, the dynamic-range compression technique is an image processing technique for preferably reproducing an HDR image having a wide dynamic range, with a display and output apparatus having a narrow dynamic range. According to these HDR techniques, highlight and shadow detail losses in a photographed image can be reduced. Various dynamic-range compression methods have been proposed, and, for example, the document “Kuang, J., Johnson, G. M., and Fairchild M. D., “iCAM06: A refined image appearance model for HDR image rendering”, Journal of Visual Communication, 2007” describes a dynamic-range compression method that reproduces a real scene the way it looks to the eye.
However, among users of conventional cameras, acceptance of photographic reproduction using the above-described HDR technique is lower than that of photographic reproduction using conventional cameras. More specifically, although a highlight portion may have a high dynamic range, the image may seem strange to a user, for example, the exposure may seem to have been altered, a dull expression may be provided, the color may seem to have been altered, or the saturation in a night scene image may appear lower.